1. Field of the Invention
The present invention relates to antennas, and particularly to a dual-band antenna used in wireless local area network (WLAN) devices.
2. Prior Art of the Invention
WLAN communication protocols mainly comprise two standards: IEEE 802.11a and IEEE 802.11b. The working frequency band of IEEE 802.11b is 2.4˜2.5 GHz. The working frequency band of IEEE 802.11a covers the range 5.15˜5.825 GHz, and comprises 5.15˜5.25 GHz, 5.25˜5.35 GHz and 5.725˜5.825 GHz.
In order to make wireless communication devices compatible for both the 802.11a and 802.11b standards, dual-band or multi-band antennas are required. The Planar Inverted-F Antenna (PIFA) is a kind of small-sized and built-in antenna currently employed in mobile communication devices. However, the electrical volume of the antenna divided by (frequency×gain×efficiency) is a constant. Therefore if the antenna is downsized, the frequency and efficiency thereof are correspondingly reduced. Therefore, the planar inverted-F antenna cannot operate in the three frequency bands of the IEEE 802.11a standard.
One solution for this problem is integrating two or more antennas into one, with each antenna working in one of the frequency bands. An example of such an integrated antenna is disclosed in U.S. Pat. No. 6,204,819 issued on Mar. 20, 2001. The integrated antenna comprises a planar inverted-F antenna and a ring antenna. The integrated antenna can be switched between the planar inverted-F antenna and the ring antenna by selecting different signal feeding means. Because it employs different antenna structures, the integrated antenna can operate in a wider frequency band. For example, certain parameters of the ring antenna can be configured to obtain a wider frequency band. However, the integrated antenna is substantially three-dimensional and occupies a relatively large space, which makes it unsuitable for low profile and small sized applications. In addition, because different signal feeding means are needed to switch between working frequency bands, the integrated antenna has a complicated structure and high costs.